Uninterruptible power supplies or systems (commonly referred to as UPS) are used to provide back-up power to critical loads such as computer systems where a loss of line power can result in the interruption of programs and the loss of valuable data. Uninterruptible power supplies may also provide a line power conditioning function to ensure that transient spikes, low-voltage conditions, or distorted power waveforms on the AC power system do not disturb the operation of the computer which is supplied with power through the UPS. Typically, the UPS includes a battery which is interfaced through an inverter to the AC output line. One type of UPS operates such that when a fault occurs in the input AC power, the inverter is controlled to provide power from the battery to the A output line at the same frequency and with substantially the same waveform as the normal input AC power. It is desirable that the switching at the time of fault is accomplished as smoothly as possible so that substantial transient spikes or dips in the waveform supplied to the AC output line do not occur.
In double conversion UPS systems, the AC power is rectified to a DC voltage which is applied, in parallel with the battery voltage, to a constantly running inverter. If the inverter fails or is otherwise unable to supply power to the load, and AC power is still available from the AC power system, a bypass is usually provided around the UPS which is switched in to supply line power directly to the load. Ordinarily, however, the inverter runs constantly so that line power is not being directly connected to the load. If the line power fails, the output inverter continues to operate, but now delivers power from the back-up power source (usually a battery) to the critical load. The input rectifier is normally able to prevent back flow of power from the battery into the main power system, but in some cases a relay may be provided to disconnect the rectifier from the AC power mains during a power outage or a brownout to electrically isolate the battery and inverter from the input terminals to the UPS and thus to the AC power mains. Because the converter is constantly running and supplying power to the load, rapid switching to disconnect the power mains from the UPS upon power failure is not necessry to avoid interruptions or glitches in the power supplied to the load. Rapid switching is required, however, if the inverter itself fails and it is necessary to switch the load directly to the AC power mains, with the typical result that a momentary interruption in power supplied to the load occurs. Because the inverter in a double conversion UPS is operating constantly, energy is constantly consumed by the UPS itself. The inverter must also be sized and rated to operate constantly under all load conditions, thus requiring costly components.
In UPS systems which utilize a ferroresonant transformer, the inverter need not be running constantly since power is normally delivered from the AC power mains through the ferroresonant transformer to the load, with the ferroresonant transformer providing transient filtering of input line power and some compensation of short power disturbances. Upon detection of a power outage or brown-out on the AC power lines, an AC switch can be opened to disconnect the primary of the ferroresonant transformer from the AC power mains and an inverter is turned on to supply power from a battery to an auxiliary primary of the ferroresonant transformer which then takes over the job of supplying the power to the load. Because of the energy storage and waveform smoothing characteristics of the ferroresonant transformer, the switching events, if properly timed, will not significantly affect the waveform of the output voltage provided from the secondary of the ferroresonant transformer to the load, resulting in "no break" power to the load. An example of a ferroresonant UPS system is shown in U.S. Pat. No. 4,692,854 to Richard V. Baxter, et al. entitled Method and Apparatus for Modulating Inverter Pulse Width.
In certain other types of back-up power supplies, the AC power mains are normally connected directly to the load, and an inverter is turned on to supply power to the load typically only when the AC power mains fail. An advantage of such systems is that the direct connection of the AC power lines to the load during normal operation avoids energy loss in the auxiliary power supply and allows relatvely less expensive and less complicated inverter components to be used since the inverter will be operated infrequently. However, because the AC power mains are ordinarily directly connected to the load, upon failure of the AC power mains the auxiliary power system must first disconnect the load from the AC power system and then turn on the inverter to supply power to the load from the back-up power source, e.g., a battery. The switching time required can extend over a significant portion of a 50 Hz or 60 Hz power system cycle, or even several cycles, often resulting in a momentary disruption of power supplied to the load, or at the least a significant distortion of the waveform of the power supplied to the load. For safety reasons, the battery and inverter of the auxiliary power supply must be electrical isolated from the AC power mains by mechanical switches, which unavoidably have relatively slow switching times as compared to a 60 Hz waveform cycle. Further, the opening of the relay switches while power current is still being supplied through them from the AC power system to the load (as during a brown-out as opposed to a total power failure, or where a glitch or waveform distortion condition is occurring in the AC power system which requires disconnection of the power system from the load) arcing, sticking or even welding of the mechanical relay contacts can occur as they are rapidly opened while carrying current in an attempt to disconnect the load from the power system.
In a UPS system in which the inverter is not constantly running to supply power to the load, the ability to rapidly and accurately detect failure of the AC power system voltage, while avoiding unnecessary operation of the UPS, is a significant consideration. Generally, it is desirable if the UPS can detect power outages within a fraction of a 60 Hz half-cycle so that the UPS can be switched to supply power to the load within the same half cycle as that in which the power loss from the AC power system was detected. An adaptive system for detecting power outages in UPS systems is described in United States patent application Ser. No. 07/404,902, filed Sept. 8, 1989 by Richard V. Baxter, Jr. et al., entitled Method and Apparatus for Line Power Monitoring for Uninterruptible Power Supplies, now U.S. Pat. No. 5,229,651, the disclosure of which is incorporated herein by reference. The system disclosed in that patent application adaptively produces a reference waveform to which the present AC power system waveform is compared in a microprocessor. The reference waveform is a composite of data from a series of prior waveform cycles which changes at a relatively slow rate so that a moderate change in the voltage waveform indicative of a fault will be detected, while the reference can nonetheless adapt and conform to systematic distortions of the line power which deviate from a pure sine wave, but which are not indicative of power line failure, so as to avoid unnecessary switching of the UPS.